Method and apparatus for distributed power generation

ABSTRACT

A method and apparatus for generating AC power. In one embodiment, the apparatus comprises a DC/AC inversion stage capable of generating at least one of a single-phase output power, a two-phase output power, or a three-phase output power; and a conversion control module, coupled to the DC/AC inversion stage, for driving the DC/AC inversion stage to selectively generate the single-phase output power, the two-phase output power, or the three-phase output power based on an input power to the DC/AC inversion stage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. provisional patent applicationSer. No. 61/343,481, filed Apr. 29, 2010, which is herein incorporatedin its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present disclosure relate generally to powerconversion and, in particular, to operating a multi-phase DC/ACinverter.

2. Description of the Related Art

Use of distributed generators (DGs) to produce energy from renewableresources is steadily gaining commercial acceptance due to the rapiddepletion of existing fossil fuels and the increasing costs of currentmethods of generating power. One such type of DG may be a solar powersystem comprising a plurality of photovoltaic (PV) modules that convertsolar energy received from the sun into a direct current (DC). One ormore inverters then convert the DC current from the PV modules into analternating current (AC). The AC power generated may then be used to runappliances at a home or business, or may be sold to the commercial powercompany.

In some DG systems, one or more inverters may each generate multi-phaseAC power. For example, an inverter may comprise a three-phase H-bridgefor converting the DC input power to a three-phase AC output power. Whenthese inverters operate during periods of reduced input power from thePV modules, such as at sunset or when the sun is obscured by a cloud, arelatively large amount of energy is expended operating all legs of thethree-phase H-bridge to achieve the three-phase output power. As aresult, the inverter suffers from an inefficient power conversion.

Therefore, there is a need in the art for a method and apparatus forefficiently operating a multi-phase DC/AC inverter.

SUMMARY OF THE INVENTION

Embodiments of the present invention generally relate to a method andapparatus for generating AC power. In one embodiment, the apparatuscomprises a DC/AC inversion stage capable of generating at least one ofa single-phase output power, a two-phase output power, or a three-phaseoutput power; and a conversion control module, coupled to the DC/ACinversion stage, for driving the DC/AC inversion stage to selectivelygenerate the single-phase output power, the two-phase output power, orthe three-phase output power based on an input power to the DC/ACinversion stage.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a block diagram of a power generation system in accordancewith one or more embodiments of the present invention;

FIG. 2 is a block diagram of an inverter in accordance with one or moreembodiments of the present invention;

FIG. 3 is a block diagram of a controller in accordance with one or moreembodiments of the present invention;

FIG. 4 is a flow diagram of a method for operating a multi-phase DC/ACinverter in accordance with one or more embodiments of the presentinvention; and

FIG. 5 is a flow diagram of a method for operating a plurality of DC/ACinverters to generate multi-phase AC power in accordance with one ormore embodiments of the present invention.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a power generation system 100 (“system100”) in accordance with one or more embodiments of the presentinvention. This diagram only portrays one variation of the myriad ofpossible system configurations. The present invention can function in avariety of environments and systems.

The system 100 comprises a plurality of PV modules 102-1, 102-2, and102-3, collectively referred to as PV modules 102, and a plurality ofDC/AC inverters 104-1, 104-2, and 104-3, collectively referred to asinverters 104. Each inverter 104-1, 104-2, and 104-3 is coupled in aone-to-one correspondence to a PV module 102-1, 102-2, and 102-3,respectively. Each inverter 104-1, 104-2, and 104-3 is further coupledin a one-to-one correspondence to a phase rotation circuit 106-1, 106-2,and 106-3, respectively; the phase rotation circuits 106-1, 106-2, and106-3 are collectively referred to as phase rotation circuits 106. Thephase rotation circuits 106 couple AC power from the inverters 104 to anAC bus 112, and, along with the PV modules 102, 106 form a branchcircuit 120 that couples three-phase AC power to the load center 110.

In some embodiments, a DC/DC converter may be coupled between each PVmodule 102 and each inverter 104 (e.g., one converter per PV module102). Alternatively, one or more of the inverters 104 may be coupled tomultiple PV modules 102; for example, the PV modules 102 may all becoupled to a single, centralized inverter 104. In some such embodiments,a DC/DC converter may be coupled between each PV module 102 and thecorresponding inverter(s) 104. In some alternative embodiments, theinverters 104 may receive DC power from a DC source other than the PVmodules 102.

The phase rotation circuits 106 couple the inverters 104 to power linesL1, L2, L3, and N at the load center 110 via the AC bus 112. The powerlines L1, L2, L3, and N are power lines of a three-phase AC powersystem; power lines L1, L2, and L3 are live conductors each carrying adifferent phase of AC power, and N is a neutral conductor. Each inverter104 comprises four inverter output lines, P1, P2, P3, and PN, forproviding AC output power (i.e., inverter 104-1 comprises inverteroutput lines P1-1, P2-1, P3-1, and PN-1; inverter 104-2 comprisesinverter output lines P1-2, P2-2, P3-2, and PN-2; and inverter 104-3comprises inverter output lines P1-3, P2-3, P3-3, and PN-3). The phaserotation circuits 106 couple the inverter output lines PN to the neutralline N. Additionally, the phase rotation circuits 106 couple theinverter output lines P1, P2, and P3 in “rotated” configurations to thepower lines L1, L2, L3 at each inverter 104. For example, the phaserotation circuit 106-1 couples inverter output lines P1-1, P2-1, andP3-1 to power lines L1, L2, and L3, respectively; the phase rotationcircuit 106-2 couples inverter output lines P1-2, P2-2, and P3-2 topower lines L3, L1, and L2, respectively; and the phase rotation circuit106-3 couples inverter output lines P1-3, P2-3, and P3-3 to power linesL2, L3, and L1, respectively. The phase rotation circuits 106 thusrotate the phases of AC power coupled to the AC bus 112 between theinverters 104. Accordingly, three of the phase rotation circuits 106coupled in series act to completely rotate the three phases of AC powerin a three-phase AC power system as depicted in FIG. 1.

One example of such a phase rotation circuit may be found in commonlyassigned, U.S. Pat. No. 7,855,473 entitled “Apparatus for Phase Rotationfor a Three-Phase AC Circuit” and issued Dec. 21, 2010, which is hereinincorporated in its entirety by reference. In some alternativeembodiments, the phase rotation circuits 106 may be contained within theinverters 104 (i.e., each inverter 104 comprises a single phase rotationcircuit 106). In some other alternative embodiments, the phase rotationcircuits 106 may be excluded from the system 100 and each inverter 104may be coupled to lines L1, L2, and L3 such that the same phase rotatingeffect between the inverters 104 is achieved.

A controller 108 is also coupled to the power lines L1, L2, L3, and N atthe load center 110; alternatively, the controller 108 may be coupled toa subset of the power lines L1, L2, L3, and N depending upon the powerrequirements of the controller 108. The controller 108 providesoperational control of the inverters 104, for example by communicatingwith the inverters 104 via power line communication (PLC) and/or othertypes of wired and/or wireless communication techniques. The controller108 may further be communicatively coupled to a master controller (notshown) for sending information to/receiving information from the mastercontrol pertaining to operation of the inverters 104.

The inverters 104 are voltage source inverters (VSI) that convert DCpower from the PV modules 102 to commercial power grid compliant ACpower and then couple the AC power (i.e., an AC current) to the loadcenter 110. The generated AC power may be further coupled from the loadcenter 110 to one or more appliances (e.g., at a private residence orbusiness) and/or to the commercial power grid. Additionally oralternatively, energy generated by the inverters 104 may be stored forlater use; for example, the generated energy may be stored utilizingbatteries, heated water, hydro pumping, H₂O-to-hydrogen conversion, orthe like. In some other embodiments, the inverters 104 may be currentsource inverters (CSI), matrix inverters, cyclo-converters, or the like.

In accordance with one or more embodiments of the present invention, theinverters 104 each generate one, two, or three phases of AC power basedon available power from the corresponding PV module 102. Each of theinverters 104 independently determines the number of phases of AC powerthat it generates, and each of the inverters 104 may generate one, two,or three phases of AC power independent of the number of phases of powerbeing generated by the remaining inverters 104. For example, whenreceiving a low power level from a corresponding PV module 102 (e.g., aninput power to the inverter 104 is less than a first threshold), aninverter 104 operates in a first phase quantity mode and generates asingle phase of output power. When receiving a medium power level fromthe corresponding PV module 102 (e.g., an input power to the inverter104 is between the first and a second threshold), the inverter 104operates in a second phase quantity mode and generates two phases ofoutput power. When receiving a high power level from the correspondingPV module 102 (e.g., an input power to the inverter 104 exceeds thesecond threshold), the inverter 104 operates in a third phase quantitymode and generates three phases of output power. The first and secondthresholds may be determined based on a maximum admissible voltageripple across the PV module 102 as determined by, for example, DCvoltage and current of the PV module 102, values of one or morecapacitors within the inverter 104, and frequency of the AC line towhich the inverter 104 is coupled (e.g., the commercial power gridfrequency). The decision point is built around maximum power conversionefficiency and maximum energy harvest; by reducing the number of phasesgenerated when lower input power is received, less energy is expended indriving switches within the inverter 104 as described further below. Inorder to determine an optimum solution, losses may be computed either inreal time or once and for all, and efficiency curves may be compared inall cases to select the best solution. In some embodiments, power may bethe main input to this decision; additionally or alternatively, DC andgrid voltages may be used as part of the decision.

In some alternative embodiments, the inverters 104 may be controlledsuch that each inverter 104 generates the same number of phases of ACoutput power.

In some embodiments, when an inverter 104 is operating to generate asingle phase of output power, the inverter 104 may operate in a burstmode when the available power from the corresponding PV module 102becomes low enough to satisfy a burst mode threshold. When operating insuch a burst mode, the inverter 104 stores energy over one or more ACline cycles and subsequently “bursts” the stored energy to the outputline. One example of burst mode operation may be found in commonlyassigned U.S. Pat. No. 7,768,155, entitled “Method and Apparatus forImproved Burst Mode During Power Conversion” and issued Aug. 3, 2010.

Each inverter 104 evaluates available power from the corresponding PVmodule 102 every commercial power grid cycle for determining anappropriate phase-quantity mode (i.e., the number of phases of power tobe generated); alternatively, available power may be evaluated more orless frequently for determining the phase-quantity mode. Afterevaluating the available power, each inverter 104 may then update (i.e.,initiate, continue, or terminate) each phase of its output power toachieve the desired number of phases. In some embodiments, each phasemay be updated every 2-10 grid cycles. Each phase of output power may beupdated accordingly at its zero crossing (for example, power generationon a particular phase may cease at a zero-crossing for that phase);alternatively, all phases of output power from an inverter 104 may beupdated simultaneously.

In some embodiments, such as the embodiment depicted in FIG. 1, theinverters 104 each generate the various phases of power on the samerespective inverter output lines P1, P2, and P3. For example, whengenerating a single phase of power, the inverters 104 each generate theoutput power on inverter output line P1; when generating two phases ofpower, the inverters 104 each generate the output power on inverteroutput lines P1 and P2. The phase rotation performed by the phaserotation circuits 106 acts to maintain a substantially balancedthree-phase power from the branch circuit 120 regardless of the numberof phases out output power generated by the inverters 104. For example,if the inverters 104 are each generating a single phase of power, thephase rotation circuits 106 distribute the power to each of the powerlines L1, L2, and L3.

In other embodiments, the phase rotation circuits 106 may be excludedfrom the system 100 and the phase rotation effect may be achieved by analternative technique in order to ensure a substantially balancedthree-phase output from the branch circuit 120. In some suchembodiments, the inverters 104 may generate output power on the samerespective inverter output lines and each inverter's output lines arecoupled to the power lines in a “rotated” configuration from theprevious inverter 104. For example, inverter output lines P1-1/P2-2/P3-3are coupled to L1; P2-1/P3-2/P1-3 are coupled to L2; and P3-1/P1-2/P2-3are coupled to L3 to ensure a substantially balanced three-phase outputfrom the branch circuit independent of the number of phases of powergenerated by the inverters 104.

In still other embodiments where the phase rotation circuits 106 are notused, the inverter output lines are coupled to the same respective powerlines and the phase rotation effect is achieved by varying the outputlines that each inverter 104 uses for generating output power; i.e.,when generating one or two phases of power, the inverters 104 eachgenerate the output on a different subset of output lines. For example,the inverter output lines P1, P2, and P3 may be coupled to power linesL1, L2, and L3, respectively, and when generating a single phase ofpower the inverters 104 each use a different output line. The outputlines on which the inverters 104 generate one or two phases of power maybe determined based on a random allocation of power output to phasenumber (e.g., based on a random word generation, such as 1 to 3, atinverter power up), a decision based on inverter serial number (e.g.,inverters having serial numbers within certain ranges utilize certainoutput lines), remote provisioning, information communicated from one ormore inverters 104, information communicated from the controller 108, orsimilar techniques. In some such embodiments, the branch circuit 120comprises a large number of inverters 104 such that some of theinverters 104 may generate a different number of phases from theremaining inverters 104 while still maintaining a substantially balancedthree-phase power from the branch circuit 120.

In some alternative embodiments, the controller 108 may determine thephase quantity mode for one or more of the inverters 104 and control theinverters 104 to generate the corresponding number of phases of power.In such embodiments, the controller 108 may determine the phase quantitymode based on information obtained from the inverters 104 (e.g., viaPLC) regarding power received from the PV modules 102. Additionally oralternatively, the controller 108 may control the inverters 104 togenerate the output power on certain inverter output lines P1, P2,and/or P3 (e.g., when an inverter 104 generates a single phase of power,the controller 108 may specify that the generated power be output oninverter output line P1). In some other alternative embodiments, theinverters 104 may communicate with one another (e.g., via PLC) fordetermining the appropriate phase quantity mode and/or determining theinverter output lines P1, P2, and/or P3 on which to generate the outputpower (e.g., a single inverter 104 may determine a phase quantity modefor one or more other inverters 104 and communicates such information tothe one or more other inverters 104).

In other alternative embodiments, a neutral line connection (i.e.,connection to line N) may not be present. In such embodiments, theinverters 104 inject power utilizing at least two of the inverter outputlines P1, P2, and/or P3.

In still other alternative embodiments, a substantially balancedtwo-phase AC power is required from the branch circuit 120 rather than athree-phase AC power. In such embodiments, the branch circuit 120generally comprises an even number of phase rotation circuit106/inverter 104 combinations, and the inverters 104 generate one or twophases of output power depending on the available PV module power.

FIG. 2 is a block diagram of an inverter 104 in accordance with one ormore embodiments of the present invention. The inverter 104 comprises aDC/DC conversion module 202 coupled across a three-phase H-bridge 220(i.e., a multi-phase DC/AC inversion stage) and a series combination ofa first capacitor 204 and a second capacitor 206. The three-phaseH-bridge 220 comprises three parallel legs, a first leg having a seriescombination of switches 208 and 210, a second leg having a seriescombination of switches 212 and 214, and a third leg having a seriescombination of switches 216 and 218. The switches 208, 210, 212, 214,216, and 218 may be metal-oxide-semiconductor field-effect transistors(MOSFETs); alternatively, switches such as junction gate field-effecttransistors (JFETs), insulated-gate bipolar transistor (IGBTs), and thelike, may be used. Inverter output lines P1, P2, and P3 are coupledbetween the switches 208/210, 212/214, and 216/218, respectively, whilethe inverter output line PN is coupled between the capacitors 204/206.In some alternative embodiments, other types of multi-phase DC/ACinversion stages may be utilized in place of the three-phase H-bridge220.

The inverter 104 further comprises DC voltage samplers 250 and 252coupled across the capacitors 204 and 206, respectively, and AC voltagesampler 222 coupled to the inverter output lines P1, P2, P3, and PN. Aconversion control module 224 is coupled to the DC/DC conversion module202, the DC voltage samplers 250 and 252, gate terminals of each of theswitches 208, 210, 212, 214, 216, and 218, the AC voltage sampler 222,and the inverter output lines P1, P2, P3, and PN.

The DC/DC conversion module 202 is coupled via two input terminals tothe PV module 102 and converts a first DC power from the PV module 102to a second DC power based on control signals from the conversioncontrol module 224. The AC voltage sampler 222 provides samples of aline voltage (e.g., commercial power grid voltage on lines L1, L2, L3,and N) to the conversion control module 224, and the conversion controlmodule 224 operates (i.e., activates/deactivates) the switches 208, 210,212, 214, 216, and 218 such that the DC power from the DC/DC conversionmodule 202 is converted to one, two, or three phases of AC power andcoupled to the commercial power grid in-phase with the line voltage. Byreducing the number of phases generated when lower input power isreceived, less energy is expended in driving the switches 208, 210, 212,214, 216, and 218.

The conversion control module 224 comprises at least one centralprocessing unit (CPU) 228 coupled to an inverter power linecommunication (PLC) transceiver 230, support circuits 226, and memory240. The inverter PLC transceiver 230 is coupled to the inverter outputlines P1, P2, P3, and N for communicatively coupling the inverter 104 toother inverters 104 and/or the controller 108 via PLC. The CPU 228 maycomprise one or more conventionally available microprocessors or digitalsignal processors (DSPs); additionally or alternatively, the CPU 228 mayinclude one or more application specific integrated circuits (ASICs).The support circuits 226 are well known circuits used to promotefunctionality of the CPU 228. Such circuits include, but are not limitedto, a cache, power supplies, clock circuits, buses, network cards,input/output (I/O) circuits, and the like. The conversion control module224 may be implemented using a general purpose processor that, whenexecuting particular software, becomes a specific purpose processor forperforming various embodiments of the present invention.

The memory 240 may comprise random access memory, read only memory,removable disk memory, flash memory, and various combinations of thesetypes of memory. The memory 240 is sometimes referred to as main memoryand may, in part, be used as cache memory or buffer memory. The memory240 generally stores the operating system (OS) 242 of the conversioncontrol module 224. The OS 242 may be one of a number of commerciallyavailable operating systems such as, but not limited to, Linux,Real-Time Operating System (RTOS), and the like.

The memory 240 may store various forms of application software, such asa conversion controller 244 for controlling the operation of theinverter 104 (e.g., controlling the DC/DC conversion module 202 and thethree-phase H-bridge 220) and a database 248 for storing data related tooperation of the inverter 104 (e.g., one or more thresholds fordetermining the phase quantity mode, burst mode thresholds, and thelike). The memory 240 may further comprise an inverter phase quantitymodule 246 for determining a phase quantity mode for the inverter 104(i.e., whether the inverter 104 generates one, two, or three phases ofAC power); additionally or alternatively, the phase quantity module 246may determine on which inverter output lines P1, P2, and/or P3 theoutput power is to be generated. The conversion controller 244 may thenoperate the three-phase H-bridge 220 to generate the appropriate numberof phases of AC power on certain inverter output lines.

The inverter phase quantity module 246 receives samples of the voltagesacross the capacitors 204 and 206 from the DC voltage samplers 250 and252, respectively (i.e., data representing the voltages across thecapacitors 204 and 206). Such samples may be received every commercialpower grid voltage cycle; alternatively, such samples may be receivedmore or less frequently. Based on the received voltage samples, theinverter phase quantity module 246 determines the phase quantity modefor the inverter 104, for example, every 2-10 grid cycles. In somealternative embodiments, a single capacitor may be coupled across thethree-phase H-bridge input and the voltage across the capacitor sampledfor determining phase quantity modes.

One or more of the voltage samples may be compared to one or morethresholds for determining the phase quantity mode. For example, whenone or more of the voltage samples are less than a first threshold, theinverter 104 operates in a first phase quantity mode and generates asingle phase of output power; when one or more of the voltage samplesare between the first and a second threshold, the inverter 104 operatesin a second phase quantity mode and generates two phases of outputpower; and when one or more of the voltage samples exceeds the secondthreshold, the inverter 104 operates in a third phase quantity mode andgenerates three phases of output power. The first and the secondthresholds may be determined based on a maximum admissible voltageripple across the corresponding PV module 102 as determined by, forexample, DC voltage and current of the PV module 102, values of thecapacitors 204 and 206, and frequency of the commercial power grid.

Additionally, the inverter phase quantity module 246 may compare one ormore of the voltage samples to a burst mode threshold for determiningwhen the inverter 104 operates in a burst mode.

In some alternative embodiments, the inverter 104 may communicateinformation regarding the phase quantity mode (e.g., the number ofphases of AC power to be generated, the inverter output lines on whichthe phases are to be generated, and the like) to one or more otherinverters 104, for example via the inverter PLC transceiver 230.

In other alternative embodiments, the inverter 104 receives informationregarding the phase quantity mode (e.g., the number of phases of ACpower to be generated, the inverter output lines on which the phaseswill be generated, and the like) from another inverter 104 or thecontroller 108, and the conversion controller 244 operates thethree-phase H-bridge 220 accordingly.

FIG. 3 is a block diagram of a controller 108 in accordance with one ormore embodiments of the present invention. The controller 108 comprisesa controller power line communication (PLC) transceiver 302, supportcircuits 306, and memory 308, each coupled to at least one centralprocessing unit (CPU) 304.

The controller PLC transceiver 302 is coupled to power lines L1, L2, L3,and N, for example at the load center 110, for communicatively couplingthe controller 108 to the inverters 104. In some alternativeembodiments, the controller 108 may utilize other wired and/or wirelesscommunication techniques for communicating with the inverters 104.

The CPU 304 may comprise one or more conventionally availablemicroprocessors. Alternatively, the CPU 304 may include one or moreapplication specific integrated circuits (ASIC). The support circuits306 are well known circuits used to promote functionality of the CPU304. Such circuits include, but are not limited to, a cache, powersupplies, clock circuits, buses, network cards, input/output (I/O)circuits, and the like. The controller 108 may be implemented using ageneral purpose processor that, when executing particular software,becomes a specific purpose processor for performing various embodimentsof the present invention.

The memory 308 may comprise random access memory, read only memory,removable disk memory, flash memory, and various combinations of thesetypes of memory. The memory 308 is sometimes referred to as main memoryand may, in part, be used as cache memory or buffer memory. The memory308 generally stores an operating system (OS) 310 of the controller 108.The OS 310 may be one of a number of available operating systems formicrocontrollers and/or microprocessors.

The memory 308 may store various forms of application software, such asinverter management software 312 for operatively controlling theinverters 104 (e.g., activating/deactivating the inverters 104). In somealternative embodiments, the memory 308 may store a controller phasequantity module 314 for determining and/or controlling the phasequantity mode of the inverters 104. In such embodiments, one or moreinverters 104 may communicate data pertaining to an input power (e.g.,input power to a multi-phase DC/AC inversion stage) to the controller108; such data may be communicated, for example, every commercial powergrid voltage cycle. Based on such data, the controller phase quantitymodule 314 may determine, for example every 2-10 grid cycles, a phasequantity mode for the inverters 104. For example, the controller phasequantity module 314 may compare at least a portion of the received datato one or more thresholds for determining the phase quantity mode asdescribed above. The controller 108 may then direct the inverters 104 tooperate in the determined phase quantity mode (i.e., to generate one,two, or three phases of power); additionally or alternatively, thecontroller 108 may direct the inverters 104 to generate the requiredoutput power on certain inverter output lines P1, P2, and/or P3. In somesuch embodiments, the controller phase quantity module 314 may determinewhen the inverter 104 operate in a burst mode; e.g., the controllerphase quantity module 314 may compare at least a portion of the receiveddata to a burst mode threshold for determining when the inverter 104operates in a burst mode.

The memory 308 may further store a database 316 for storing informationrelated to the present invention, such as data received from theinverters 104 (e.g., input power data); power thresholds for determiningthe phase quantity mode, burst mode thresholds, and the like.

FIG. 4 is a flow diagram of a method 400 for operating a multi-phaseDC/AC inverter in accordance with one or more embodiments of the presentinvention. In some embodiments, such as the embodiment described below,a DC/AC inverter comprises a three-phase DC/AC inversion stage (e.g.,inverter 104 comprising three-phase H-bridge 220) for inverting a DCinput to an AC output, where the three-phase DC/AC inversion stage maybe operated to generate one, two, or three phases of AC power. The DCinput may be received from a PV module coupled to the DC/AC inverter;alternatively, the input power may be received from any suitable DCpower source. The generated AC output power may then be coupled, forexample, to a commercial power grid.

In some embodiments, the DC/AC inverter is a voltage source inverter(VSI); alternatively, the inverter may be a current source inverter(CSI), matrix inverter, cyclo-converter, or the like.

In other embodiments, the DC/AC inverter may comprise a two-phase DC/ACinversion stage that may be operated to generate one or two phases of ACpower.

The method 400 starts at step 402 and proceeds to step 404. At step 404,an available power to the DC/AC inverter, such as input power to theDC/AC inversion stage, is determined. In some embodiments, the DC/ACinversion stage input power may be determined every commercial powergrid cycle; alternatively, the DC/AC inversion stage input power may bedetermined more or less frequently. In some embodiments, a seriescombination of two capacitors may be coupled across the DC/AC inversionstage input and a neutral line output of the inverter is coupled betweenthe capacitors. In such embodiments, DC voltages across each of thecapacitors may be sampled for determining input power. In somealternative embodiments, a single capacitor may be coupled across theDC/AC inversion stage input and the voltage across the capacitor sampledfor determining input power.

The method 400 proceeds to step 406. At step 406, the inverterdetermines a phase quantity mode for operation based on the input power.In some embodiments, the phase quantity mode may be determined every2-10 commercial power grid cycles; alternatively, the phase quantitymode may be determined more or less frequently. The phase quantity modeindicates a number of phases of AC power to be generated by the DC/ACinversion stage (e.g., one, two, or three phases). In some embodiments,the phase quantity mode may be determined by comparing the inversionstage input power to one or more thresholds. For example, a first phasequantity mode may be used to generate a single phase of AC power whenthe input power is less than a first threshold, a second phase quantitymode may be used to generate two phases of AC power when the input poweris between the first and a second threshold, and a third phase quantitymode may be used to generate three phases of AC power when the inputpower exceeds the second threshold. The first and second thresholds maybe determined based on a maximum admissible voltage ripple across the PVmodule as determined by, for example, DC voltage and current of the PVmodule, values of one or more capacitors within the inverters, andfrequency of the AC line to which the inverter is coupled (e.g., thecommercial power grid frequency). The decision point is built aroundmaximum power conversion efficiency and maximum energy harvest; byreducing the number of phases generated when lower input power isreceived, less energy is expended in driving switches within theinverter. In order to determine an optimum solution, losses may becomputed either in real time or once and for all, and efficiency curvesmay be compared in all cases to select the best solution. In someembodiments, power may be the main input to this decision; additionallyor alternatively, DC and grid voltages may be used as part of thedecision.

In some alternative embodiments, the inverter may communicate thedetermined mode to one or more other inverters; additionally oralternatively, the inverter may communicate information to one or moreother inverters indicating on which inverter output lines the powershould be generated. In other alternative embodiments, a centralizedcontroller (e.g., the controller 108) may determine the phase quantitymode and communicate the determined mode to the inverter. In suchembodiments, the centralized controller may receive information from theinverter, for example the samples of the input power to the DC/ACinversion stage, for determining the phase quantity mode. The controllermay also communicate information to the inverter to indicate on whichinverter output lines the power should be generated.

The method 400 proceeds to step 408. At step 408, the DC/AC inversionstage is operated to generate one, two, or three phases of AC powerbased on the phase quantity mode. As described above, the generatedphase or phases of power may be output on fixed inverter output lines.Alternatively, the generated phases may be output on inverter outputlines as determined by the inverter, for example, based on a randomallocation of power output to phase number (e.g., based on a random wordgeneration, such as 1 to 3, at inverter power up), a decision based oninverter serial number (e.g., inverters having serial numbers withincertain ranges utilize certain output lines), remote provisioning, orsimilar techniques. In some alternative embodiments, the output linesfor couple one or two phases of output power may be specified by anotherinverter or the controller.

In some embodiments, when an inverter is operating to generate a singlephase of output power, the inverter may operate in a burst mode if theavailable power becomes low enough to satisfy a burst mode threshold(e.g., when it is determined that the inversion stage input powersatisfies a burst mode threshold). When operating in such a burst mode,the inverter stores energy over one or more AC line cycles andsubsequently “bursts” the stored energy to the output line. In some suchembodiments, the inverter may determine when the input power satisfiesthe burst mode threshold for operating in burst mode. In somealternative embodiments, the controller may make such a determinationand communicate information to the inverter for operating in the burstmode.

The method 400 proceeds to step 410, where a determination is madewhether to continue operating the inverter. If the result of suchdetermination is yes, the method 400 returns to step 404. In someembodiments, the input power is compared to one or more thresholds every2-10 commercial power grid cycles for determining the phase quantitymode for the inverter. If, at step 410, a determination is made to notcontinue operating the inverter, the method 400 proceeds to step 412where it ends.

FIG. 5 is a flow diagram of a method 500 for operating a plurality ofDC/AC inverters to generate multi-phase AC power in accordance with oneor more embodiments of the present invention. In some embodiments, suchas the embodiment described below, a plurality of DC/AC inverters arecoupled in a branch circuit configuration for providing a three-phase ACpower to a load center (e.g., as in the power generation system 100).The DC/AC inverters each comprise a multi-phase DC/AC inversion stagefor inverting a DC input to an AC output (e.g., inverters 104 comprisingthree-phase H-bridge 220). The multi-phase DC/AC inversion stages mayeach be operated to produce one, two, or three phases of AC power basedon a phase quantity mode for the corresponding inverter. In somealternative embodiments, the branch circuit may provide a two-phase ACpower to a load center and the multi-phase DC/AC inversion stages areeach operated to produce one or two phases of AC power based on thephase quantity mode.

In some embodiments, the DC/AC inverters are voltage source inverters(VSI); alternatively, the inverters may be current source inverters(CSI), matrix inverters, cyclo-converters, or the like.

The method 500 starts at step 502 and proceeds to step 504. At step 504,one or more phase quantity modes for operating the inverters aredetermined, for example, as described above with respect to the method400. The phase quantity modes may be determined every 2-10 commercialpower grid cycles; alternatively, the phase quantity mode may bedetermined more or less frequently. Each of the inverters independentlydetermines a phase quantity mode for operation based on the input powerto its DC/AC inversion stage, as previously described. In somealternative embodiments, a single phase quantity mode for operating allof the inverters may be determined. In some such embodiments, one of theinverters may determine the phase quantity mode and may communicate thedetermined mode to one or more of the other inverters. In other suchembodiments, a centralized controller (e.g., the controller 108) maydetermine the phase quantity mode and communicate the determined mode tothe inverters, where the centralized controller may receive informationfrom the inverters, for example samples of the voltages across the DC/ACinversion stages, for determining the phase quantity mode.

The method 500 proceeds to step 506. At step 506, the DC/AC inversionstages are operated to generate one, two, or three phases of AC powerbased on the determined phase quantity mode for each inverter. For eachinverter, each phase of output power may be updated accordingly at itszero crossing (for example, power generation on a particular phase maycease or begin at a zero-crossing for that phase); alternatively, allphases of output power may be updated simultaneously. In someembodiments, when an inverter is operating to generate a single phase ofoutput power, the inverter may operate in a burst mode when theavailable power becomes low enough to satisfy a burst mode threshold aspreviously described. The method 500 proceeds to step 508.

At step 508, the generated power is coupled to the appropriate lines ofthe branch circuit for providing a substantially balanced three-phase ACpower from the branch circuit. In some embodiments, the inverters eachgenerate power on the same respective inverter output lines and a phaserotation technique is utilized to rotate the phases of the power linesbetween inverters. The phase rotation technique couples the inverteroutput lines for subsequent inverters to different live conductor powerlines than the preceding inverter; e.g. inverter output lines P1-1,P1-2, and P1-3 are coupled to power lines L1, L3, and L2, respectively.Thus, when the inverters each generate power on the same respectiveinverter output lines, the branch circuit provides three-phase AC powerregardless of the number of phases of power being generated by eachinverter. Such a phase rotation among inverters may be achieved byutilizing phase rotation circuits, such as the phase rotation circuits106. Alternatively, the phase rotation may be achieved by directlycoupling the inverter output lines to the power lines in the appropriateconfigurations; i.e., each inverter's output lines are coupled to thepower lines in a “rotated” configuration from the previous inverter.

In other embodiments, the inverter output lines from each inverter arerespectively coupled to the same live conductor power lines and, whengenerating one or two phases of power, the inverters each generate theoutput power on a different subset of output lines in order to achieve asubstantially balanced three-phase output from the branch circuit. Forexample, the inverter output lines P1, P2, and P3 may be coupled topower lines L1, L2, and L3, respectively, and each inverter 104generates a single phase of power on a different output line P1, P2, orP3 to achieve a three-phase power from the branch circuit. In some suchembodiments, the inverter output lines to be utilized when generatingone or two phases of power are fixed; for example, an inverter alwaysutilizes inverter output line P1 when generating a single phase ofoutput power. Alternatively, each inverter may determine the outputlines on which to generate the output power based on a random allocationof power output to phase number (e.g., based on a random wordgeneration, such as 1 to 3, at inverter power up), a decision based oninverter serial number (e.g., inverters having serial numbers withincertain ranges utilize certain output lines), remote provisioning, orsimilar techniques.

In some alternative embodiments, the centralized controller may instructeach of the inverters as to which inverter output lines should beutilized for the generated output power, or one or more inverters maydetermine on which inverter output lines the power is to be generatedand may communicate such information to one or more other inverters.

In some embodiments, each of the inverters is coupled to a neutral lineof the branch circuit and each individual phase of power generated by aninverter is coupled to a single power line. In other embodiments, theinverters are not coupled to the neutral line and the inverters injectpower on at least two of the power lines.

The method 500 proceeds to step 510, where a determination is madewhether to continue operating the inverters. If the result of suchdetermination is yes, the method 500 returns to step 504. In someembodiments, a phase quantity mode for the inverters is determined every2-10 commercial power grid cycles at step 504; alternatively, the phasequantity mode may be determined more or less frequently. If, at step510, a determination is made to not continue operating the inverter, themethod 500 proceeds to step 512 where it ends.

The foregoing description of embodiments of the invention comprises anumber of elements, devices, circuits and/or assemblies that performvarious functions as described. These elements, devices, circuits,and/or assemblies are exemplary implementations of means for performingtheir respectively described functions.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

The invention claimed is:
 1. Apparatus for generating AC power,comprising: a DC/AC inversion stage capable of generating at least twoof a single-phase output power, a two-phase output power, and athree-phase output power; and a conversion control module, coupled tothe DC/AC inversion stage, for driving the DC/AC inversion stage toselectively generate a number of phases of output power from the atleast two of the single-phase output power, the two-phase output power,and the three-phase output power based on an input power to the DC/ACinversion stage, wherein the conversion control module (i) drives theDC/AC inversion stage to generate the single-phase output power when theinput power is less than a first threshold, (ii) drives the DC/ACinversion stage to generate the two-phase output power when the inputpower is between the first threshold and a second threshold, and (iii)drives the DC/AC inversion stage to generate the three-phase outputpower when the input power exceeds the second threshold, and wherein theconversion control module drives the DC/AC inversion stage to operate inburst mode when the input power satisfies a burst mode threshold.
 2. Theapparatus of claim 1, wherein the first and the second thresholds aredetermined based on a maximum admissible voltage ripple across a DCpower source.
 3. The apparatus of claim 1, wherein the burst modethreshold is less than the first threshold.
 4. The apparatus of claim 1,wherein the conversion control module determines, based on at least oneof a random number generation or a serial number, one or more outputlines on which to generate the single-phase output power or thetwo-phase output power.
 5. A method for generating AC power, comprising:determining an input power to a DC/AC inversion stage, wherein the DC/ACinversion stage is capable of generating at least two of a single-phaseoutput power, a two-phase output power, and a three-phase output power;driving the DC/AC inversion stage to selectively generate a number ofphases of output power from the at least two of the single-phase outputpower, the two-phase output power, and the three-phase output powerbased on the input power, wherein driving the DC/AC inversion stagecomprises (i) comparing the input power to at least one of a first or asecond threshold, (ii) driving the DC/AC inversion stage to generate thesingle-phase output power when the input power is less than the firstthreshold, (iii) driving the DC/AC inversion stage to generate thetwo-phase output power when the input power is between the first and thesecond threshold, and (iv) driving the DC/AC inversion stage to generatethe three-phase output power when the input power exceeds the secondthreshold; determining whether the input power satisfies a burst modethreshold; and driving the DC/AC inversion stage to operate in burstmode when the input power satisfies the burst mode threshold.
 6. Themethod of claim 5, further comprising determining the first and thesecond thresholds based on a maximum admissible voltage ripple across aDC power source.
 7. The method of claim 5, wherein the burst modethreshold is less than the first threshold.
 8. The method of claim 5,further comprising determining one or more output lines on which togenerate the single-phase output power or the two-phase output power. 9.The method of claim 8, wherein the one or more output lines aredetermined based on at least one of a random number generation or aserial number.
 10. A system for generating AC power, comprising: abranch circuit, comprising: a plurality of photovoltaic (PV) modules;and a plurality of inverters, coupled to the plurality of PV modules ina one-to-one correspondence, wherein each inverter of the plurality ofinverters comprises (i) a DC/AC inversion stage capable of generating atleast two of a single-phase output power, a two-phase output power, anda three-phase output power, and (ii) a conversion control module,coupled to the DC/AC inversion stage, for driving the DC/AC inversionstage to selectively generate a number of phases of output power fromthe at least two of the single-phase output power, the two-phase outputpower, and the three-phase output power based on an input power to theDC/AC inversion stage, wherein the conversion control module (i) drivesthe DC/AC inversion stage to generate the single-phase output power whenthe input power is less than a first threshold, (ii) drives the DC/ACinversion stage to generate the two-phase output power when the inputpower is between the first and a second threshold, and (iii) drives theDC/AC inversion stage to generate the three-phase output power when theinput power exceeds the second threshold, and wherein the conversioncontrol module drives the DC/AC inversion stage to operate in burst modewhen the input power satisfies a burst mode threshold.
 11. The system ofclaim 10, further comprising a plurality of phase rotation circuitscoupled to the plurality of inverters in a one-to-one correspondence,wherein the plurality of phase rotation circuits rotate phases of powerfrom the plurality of inverters among power lines of the branch circuitto produce a substantially balanced three-phase power from the branchcircuit.
 12. The system of claim 10, wherein the first and the secondthresholds are determined based on a maximum admissible voltage rippleacross a DC power source.
 13. The system of claim 10, wherein the burstmode threshold is less than the first threshold.
 14. The system of claim10, wherein the conversion control module determines, based on at leastone of a random number generation or a serial number, one or more outputlines on which to generate the single-phase output power or thetwo-phase output power.